A maneuver for fucking that uses a sling. The person fucking the person in the sling lifts himself up using the chains on a sling or the frame of the sling and then uses the inertia of the sling rocking back and forth to fuck the slingee.

That person just tried the Fenn he must have good upper body strength.

Fenn is usually used in conjuncture with the word 'that', therefore making it 'fenn that'. Its means; give me that, lend that to me, give a bit of that etc.
9 times outta 10 its used to ask for tokes on a zoot a.k.a spliff.